In solvents, either a non-protogenic, often referred to as “aprotic”, liquid or polymers bearing polar groups such as ethers, esters or nitriles, there is no solvation of the negative X− charge when attempting to induce ion conduction, i.e. forming an electrolyte, by dissolving a salt X−M+ in such media. Solubility then dissociation of the ion pair X−M+ is thus only effective when the anion X− does not require stabilization by forming hydrogen bonds, as in water, alcohols or amides RCONHR′ where R or R′═H, an organic radical. Thus the main anions used in practice are ClO4−, BF4−, PF6−, AsF6−, SbF6−, RfSO3−, [(RfSO2)2N]−, and (RfBF3)− where Rf═CnF2n+1, (0≧n≧8). Another family of anions are the so-called “Hückel anions”, such as 2-trifluoromethyl-4,5-dicyano-imidazole.
A main driving force for designing new anions is in the field of lithium batteries, in particular for highly conductive electrolytes where the polarizing lithium M+=Li is the countercation. The electrolyte in lithium batteries is subjected to extreme oxidizing conditions on the positive electrode side and extreme reducing conditions on the negative electrode side. ClO4− leads to explosive mixtures with organic solvents and polymers; BF4− and RfSO3− lead to a poorly conductive solution due to ion pairing; and AsF6− and SbF6− have as a core a heavy, highly toxic element. [(RfSO2)2N]− or its higher homologues {CF3SO2N[S(O)(CF3)N]nSO2CF3}− are resistant to oxidation, but they do not passivate aluminum which is the only affordable positive electrode current collector. Most lithium batteries use LiPF6 as a solute, which is far from satisfactory mainly since the salt has a propensity to equilibrate as LiPF6⇄LiF+PF5, the latter being a highly reactive Lewis acid, progressively destroying the solvent undergoing carbocationic chemistry. Another problem with all the salts containing fluorine is the release of highly toxic HF in the case of fire, an incident that occurs presently at a rate of a few ppm/cell, unacceptable for large scale applications, like electric road transportations. Presently, it has been impossible to avoid using fluorine, the most electronegative element, to impart resistance to oxidation to the anions. For instance the Bis(oxalatoborate) anion {[(C2O4)2]B}− evolves CO2 above 4 Volts vs. Li+:Li°.